This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. A critical step in protein biosynthesis involves either translocation across a cellular membrane or integration into a cellular membrane. Both processes proceed via the Sec translocon - a ubiquitous and highly conserved transmembrane channel. Recent structural studies offer high-resolution snapshots of the translocon, and a wealth of biochemical and genetic data reveal important interactions within the translocon;but many aspects of its mechanism and regulation remain unclear. We propose to address two central questions about the mechanism of Sec-facilitated protein translocation and membrane integration that are nearly impossible to resolve without the long-timescale simulations made possible by Anton. Specifically, these questions are: (1) What is the conformational response of the Sec translocon to the slow insertion of hydrophobic and hydrophilic peptide substrates? (2) Does the binding of molecular motors to the translocon induce conformational changes that are essential for protein translocation and membrane integration? We propose long-timescale simulation studies that will directly address these two fundamental questions.